Angharad, pre covid medical schools were expected to get their numbers right. Too many or too few, and they got fined. They were normally reasonably good at predicting how many might miss and to what extent they needed to over offer. If too many missed grades they might accept some anyway, and some maintained discrete waiting lists or, on occasion went back to applicants they had previously rejected. SGUL was unusual in that it would go into clearing. There were also stories of medical schools who had gone over numbers offering students financial incentives to defer.
So it depends. DD was surprised when she started at how many of her peers had missed grades and still got in.
Last year was chaos. Virtually no one missed grades, and a number of medical schools ended up with too many. This creates on-going problems with spaces in lecture rooms, GP placements, clinical placements and so on. The last thing a medical school wants this year is too many achieved offers. It is reasonable to expect that they are being cautious and will fill any spare places with either waiting lists, which seem to be much more of a thing this year. Or by allowing people in with missed grades, or in some instances through clearing. As in pre-covid years, it really isn't over until term starts.
Resit and reapplication is very possible. Some research is needed as to which medical schools allow resits, but medical school websites/Student Room should have the answers.
Your point about Cardiff offering interviews in Welsh is interesting. Inter alia, non English mother tongue students, however fluent, tend to do less well at UCAT. DD is at Bristol which has a lot of students from Wales. One reason they give for preferring Bristol to Cardiff is that placements are less spread out. Cardiff has placements all over Wales, some of which are hard to reach.
Good luck.